22 November, 2014

Millions of years ago (somewhere between 443 - 419 of them to be specific) in the Silurian period of the Paleozoic era, as the first plant life began to appear on Earth, rocks were laid down - sandstone, mudstone, shale, quartzite. These became the bedrock which underlies the Ballarat goldfields and the surrounding districts. Through this rock ran the auriferous quartz reefs which provided such fabulous wealth for the mining companies like those at Steiglitz - big companies with big machines which could sink a shaft hundreds of metres down and then follow the lead, extracting all the precious gold.
Today, this ancient Silurian rock can still be seen, exposed to the elements at points along the various watercourses of the region such as the Leigh River and its tributaries including Cargerie, Williamson's, Reid's and Coolabaghurk Creeks. It can also be seen in rather dramatic form in the steep rock walls which make up the narrow gorge through which the Leigh flows at the Leigh Grand Junction Bridge.

Silurian rock formation at the Leigh Grand Junction Bridge

In sections of the river like this, the underlying rock is exposed, however most of it lies deep underground, covered by the products of subsequent geological events - events which also helped shape the countryside we see today and the rich goldmining industry which flourished during the 19th and early 20th centuries.
As I discussed in my previous post, from the 1860s onwards as the shallow alluvial gold supplies began to dry up, the large mining companies moved in, working large claims around the Ballarat region and extending south onto the Sebastopol Plateau and beyond. These companies were all working "deep leads" - deposits of alluvial gold buried deep underground along the courses of ancient riverbeds. One such lead to the south of Ballarat was known as the Durham Lead. It was buried up to 100m below the surface, under layers of basalt laid down during volcanic events more than 3 million years ago.
This of course raises an interesting question: exactly which ancient riverbed does the Durham Lead follow?
In geological terms, the answer lies back in the Paleozoic era when the Silurian bedrock of the area was first laid down and in the millions of intervening years up to the present. During this time (over 400 million years), the forces of nature exerted their influence. The rocks were subject to weathering and erosion. Gradually, watercourses formed. These were wide, shallow streams cutting their way through the ancient bedrock, forming new rivers and creeks.

Diagram showing the relative geological timeframe of the events along the Leigh River
from 541 million years ago (the beginning of the Paleozoic era) to the present day.
NOTE: the divisions are NOT to scale
* Scale in millions of years before the present day

One of these ancient rivers carved out a path which was not dissimilar to the current course of the Leigh River as we know it today. This ancient ancestor of the modern Leigh River cut through not only the Silurian rocks described above, but also through the quartz reefs contained within them. In doing so, any gold contained with these reefs was freed from the bedrock but collected within the river channel. Being heavy, much of the gold sank and became trapped in the riverbed and according to early geological reports from the 1860s, did not move far from its original source.
Over time, temperatures changed, sea levels rose and coastlines retreated. Rivers, along with the countryside were inundated and their beds filled with the remains of marine organisms by a huge Miocene sea stretching inland as far as the township of Meredith. These calcium-rich deposits would eventually form limestone, tracts of which can still be found scattered across the region at places such as Fyansford where limestone has long been quarried. In the case of the ancient Leigh River, the Miocene deposits extend a,lmost up to the Bamganie area, covering the ancient riverbed and the "drift" which had collected along its course from earlier erosion.
Once again however, with the passing of many more millennia, the region entered a stage of deposition. Sea levels began to recede and more material was laid down during what is termed the Pliocene era. This was followed by a period of erosion where the bed of the ancient Leigh was carved deeper into the bedrock, its channel becoming steeper and the flow of the river, more powerful. These early Pliocene deposits were worn away, surviving as caps on higher outcrops in areas such as the rising ground between the modern Leigh River and Native Hut Creek or the Leigh and Williamson's Creek.

New deposits, a combination of the old Pliocene material, the bedrock and of course, alluvial gold once again settled on the riverbed. It was this material which would one day form the "wash dirt" of the Durham Lead, the mud, clay and rocks from which the prized gold had to be extracted, but that was still several million years in the future. At this time (later in the Pliocene era) the region entered a phase of volcanic activity. During this period several distinct volcanic events affected the course of the old river. The earliest, flowed down the course of the river from the north, trapping the sediment deposited on the riverbed under a thick layer of basalt which in turn was covered by another layer of drift material. A second eruption covered the three previous layers, almost filling the bed of the old river before a third eruption (possibly flowing south from Mt Mercer and Mt Lawaluk on the west and Green Hills - aka Mt Collier - to the east) overtopped the channel and spread across the plains below Mt Mercer. In this area, the course of the old Leigh River and any gold it contained was now completely buried beneath the basalt plains.

Aerial view of the Mt Mercer volcano cone. Click to enlarge

﻿﻿﻿Further down, below Bamganie, all trace of the lead was thought to have been obliterated by the encroachment of the Miocene sea. From this point, the ancient riverbed was lined not with Silurian bedrock, but with Miocene limestone.

Of course, with the filling of the old channel, the water which flowed from the higher ground above Ballarat needed to go somewhere, so over time a new channel formed, wearing its way through the recently formed upper layer of basalt. Often it flowed along the line where the hard basalt met the softer Silurian rock. In its upper reaches, the new river for the most part followed a similar course to the old, cutting back and forth across the old riverbed until it reached the Mt Mercer flow.

At this point, the lava flow was so great that the course of the old river disappeared entirely beneath the basalt and it became all but impossible for the mining companies to trace the lead. As a consequence, there was little large-scale mining activity below the Leigh Grand Junction Bridge.

For the newly formed Leigh River, it became necessary at this point to forge its own path through the basalt, which it did, winding its way towards Shelford and Inverleigh, forming what we recognise today as the course of the modern Leigh River.

12 November, 2014

Whilst researching my previous posts on the Leigh Grand Junction Bridge, I came across an interesting plan, dating back almost to the Gold Rush, to drain unwanted water from the gold mining operations which had spread from Ballarat all the way down to Mount Mercer and beyond. This rather long post looks at those plans.
By the 1850s, gold was all the rage across the newly-declared colony of Victoria and nowhere more so than in Ballarat and surrounds. Prospectors flocked to the goldfields, hopeful of making their fortune working the shallow deposits of alluvial gold which seemed to exist in such abundance.
By the 1870s however, most of the shallow leads had been worked out and gold was becoming harder to find and more expensive to mine. As a result, this decade saw the arrival of the big mining companies chasing deep leads and quartz reefs. On the low lands to the south of Ballarat, this was especially true. Alluvial gold was there to be found, but much of it lay deep in the ground under a layer of hard basalt, which had flowed from surrounding volcanoes, filling ancient creeks and riverbeds where equally ancient alluvial deposits had formed millions of years earlier. Also beneath the basalt lay the quartz reefs, the products of even earlier volcanic events.
As the mining companies moved in, they brought with them their blasting equipment, crushing batteries, water pumps and the steam engines required to power them all.

Looking across the Sebastopol workings

The pumps, whilst expensive to run, were vital to the safe operation of the mines and the deeper the mine, the more of a problem water became. Not only did it make removal of auriferous material difficult, if a shaft became suddenly inundated, it could be fatal for the miners below the surface. Whilst buckets could be used to manually remove water from shallower mineshafts with minimal seepage, deeper shafts - often with lateral tunnels known as drives extending at an angle from the main shaft - also suffered seepage and were prone to catastrophic flooding if water were to break through from the more porous layers above the bedrock and enter the mine.
Steam-driven pumps were often used to drain shafts however they were expensive to operate, requiring a constant supply of fuel to keep the engines running. As a general rule, it was only the big mining companies which could afford such equipment and this was true of the area to the south of Ballarat where even today, the names of the towns reflect those of the mines which operated in the area - names such as Napoleons, Durham Lead, Enfield, Grenville and Scotchman's Lead.
﻿

Illustration and cross-section view of the Pioneer Gold Mining Company's
mine a little to the south of today's Durham Lead township and the same
distance east of the Leigh. Image held by the State Library of Victoria.
Click to enlarge

So much of an issue was drainage, that in 1877 the Department of Mines requested a report into the feasibility of draining not just a single mine, but the entire Sebastopol Plateau and the area below that known as the Durham Lead - an area extending south from Ballarat towards Mount Mercer and covering an area of around 120 square miles (311 square kilometres) - of which at least 40 square miles (104 square kilometres) was thought to be auriferous. It was felt that draining the plateau would not only open up access to new leads, but also allow for the cheap reworking of earlier claims which either had not been efficiently mined or which had been abandoned due to flooding.
﻿The report was produced by the geological surveyor Reginald A. F. Murray who noted that the watershed of the Leigh River above the Perseverance Mine (previously the Chryseis mine situated near the Leigh about 2km north east of the modern township of Grenville) covered an area of around 200 square miles (518 square kilometres) which he claimed equated to a daily subterranean "percolation" of around 95 million gallons (approximately 360 megalitres) of water through the plateau. But how to drain it?

Map showing Ballarat and Sebastopol mining claims on the upper
section of the plateau. Image held by the State Library of Victoria.
Click to enlarge

Murray proposed a two-part process to effect the required drainage. The first and most detailed stage involved the construction of an adit: a horizontal (or almost horizontal) tunnel providing access to a mine, which could be used for ventilation, removal of minerals or drainage. He stated that the adit should run from the Perseverance Mine on the Leigh, more than 6 miles to one of two feasible points about a mile and a half below the Leigh Grand Junction Bridge and should be funded by the government at a cost of £38,000. It could be constructed in two and a half years by tunnelling from both ends and from eight shafts sunk at intervals in between. Costs would be kept low as the tunnel would not require drilling through basalt, only "Silurian rocks of the ordinary character" and where this was the case, timbering or brick lining to support the tunnel would not be required, offsetting the extra cost of blasting through the stone.
﻿﻿﻿﻿﻿

Map which accompanied Murray's report, showing the proposed line of the first section of the adit.
Image held by the State Library of Victoria. Click to enlarge.

The second stage involved extending the adit - using private finance - further north "through or along the old workings of the Durham Lead as far as may be requisite". The report seems a little vague as to exactly how much of the Plateau and Durham Lead would be effectively drained, but an article in the Bendigo Advertiser of 17th December, 1877 suggests that it would drain mining claims from the Durham Company's mine up to Ballarat. In the event, a drainage bill was introduced to parliament in October, 1878. It was withdrawn a month or so later, only to be reintroduced the following year, this time with two suggested alternatives: Murray's plan, or a scheme which involved the use of steam pumps to do the work of the adit. It would seem that nothing was resolved as the issue of draining the plateau was also in the newspapers in 1881, 1882 and again in 1886 when the Mines Department called for expressions of opinions as to how best to drain the plateau. The following year, the issue was put before a Parliamentary select committee where three proposals were tabled: 1) to drain the plateau entirely by pump 2) to extend the adit all the way up to Sebastopol or 3) to drain the lower Durham Lead area using the initial adit suggested by Murray, with pumps then being used to drain the upper section.
Many opinions were put forward for and against each option. There were those who felt that whilst cheaper, the pumps would not provide adequate drainage, whilst others suggested that Murray's estimates fell far short of what would be the true cost of constructing the adit. It was also suggested that to use only a system of adits would require around 19 miles, 10 chains of tunnel with a further 15 miles of tributary adits requiring the sinking of 38 shafts - quite a different story to Murray's proposal and significantly more expensive! Even then, the financial returns suggested by Murray were by no means guaranteed and nor was the drainage itself, it was claimed.
By November, 1889, the issue formed part of a broader royal commission into gold mining in Victoria with the same claims and counter claims about the efficacy of the suggested processes being put forward and some witnesses even claiming that there was little gold left in the area anyway.
Once again it seems no decision was reached and in 1901, 1902, 1903 and in 1905 attempts were still being made to secure government funding for some form of drainage, this time however with the added twist that "by his [the mining engineer's] scheme the adit would serve as a main sewer for the deep drainage of Ballarat and also allow for increasing the water supply."
Again, no definite action was taken although government grants were provided in some cases for private companies.
Then, finally in 1935, the media was abuzz with the news that the plan to construct a "20 mile" adit to drain the plateau had once again been proposed. This time a London company was prepared to spend what was now estimated as being the £1,000,000 required to complete the work which, the papers noted, had lapsed for the last 70 years owing to the expense. However, despite media declarations that drainage was imminent, there is no further mention of such a scheme past this time.
The plateau, it seems, was not drained, the adit was not constructed and the flow of water into the Leigh River was not changed as a consequence of such action, although it is interesting to speculate what the environmental effect of such a plan might have been.

05 November, 2014

The Leigh Grand Junction Bridge which today crosses the Leigh River east of Mount Mercer, is the second bridge to stand on this site. The 40 year history of the first bridge is described in my two previous posts here and here. The second bridge will be the focus of this post.
By 1908 it had become clear that the first bridge - built in 1873 - was in desperate need of replacement. In November that year, Buninyong Shire got the ball rolling when they approached the Leigh Shire to discuss the issue. The following month, the Leigh Shire engineer (presumably CAC Wilson) reported to council that the bridge was in bad repair and should be replaced by an iron girder bridge.
Over the next year and more, decisions were made, costs determined and most importantly, plans were drawn up. The end result however was not the iron girder structure recommended, but rather a revolutionary, steel-reinforced prestressed-concrete bridge. The technology was relatively new to Victoria as the first concrete bridge had been completed just over a decade earlier in 1899.
﻿

Charles Corbett Powell Wilson, civil engineer. Image held
by the State Library of Victoria

This revolutionary design was the brainchild of Charles Corbett Powell Wilson, shire engineer from 1908 for both Meredith and Buninyong, and then, from 1910 upon the retirement of his father CAC Wilson, for Leigh Shire also. CAC was a pioneer in the use of concrete to build bridges and no doubt passed this interest on to his eldest son.
﻿﻿ Between them, these two men were responsible for the design and construction of scores of bridges and a vast array of community facilities across the three shires. For over 90 years from 1864 to 1938 they served as engineers in one shire or another and amongst their contributions were a number of bridges which spanned the Barwon, Moorabool, and Leigh Rivers and their tributaries (post to follow). Few however, were as impressive as the Leigh Grand Junction Bridge.
﻿

By the time the bridge was completed in the first months of 1911, the total cost had been reckoned at £1,118 of which the government agreed to pay £150. The remainder of the cost was to be divided between the three shires, with Buninyong paying half and the other two shires one quarter each. As per my previous post, after significant debate, this was the eventual outcome.
As to the bridge itself, the end result was a concrete structure with a carrying capacity of 30 tons. Coming in at 165 feet in length, it was however only 8 feet wide - a single lane. Unusually for bridges in the area, it stood some 35 feet above the river below (or by today's measurements: about 50.3m long, marginally less than 2.5m wide and over 10.5m high). The significant elevation was a necessary requirement due to the Leigh River having quite a steep, narrow channel at this point.
Today, the Leigh Grand Junction Bridge is one of the oldest surviving examples of a true concrete-reinforced girder bridge in Australia and not surprisingly, is heritage listed.
﻿

Looking upstream from the picnic area

Structurally, the bridge has four spans, each 38 feet 6 inches in length, which are supported by three slender piers and at its northern end, by a bluestone abutment, with concrete serving at the southern end. The columns of these piers are reinforced with "mild" or low-carbon steel (suitable for many purposes, including bridge-building) and are further strengthened by interconnecting diaphragms (concrete panels) at two levels.
In a clever move of both economic efficiency and structural enhancement, the steel used to reinforce the four T-section, concrete girders which span the bridge was recycled cable from the Melbourne tramways. The girders are connected to the piers below by distinctive triangular fillets and the whole is topped by a concrete slab sealed with bitumen, flanked by metal guard rails.
Protection during times of flooding is provided by cut-waters - triangular projections on the upriver face of the outermost column of each pier, which reduce pressure on the bridge during times of high water flow.

Bridge detail. Click to enlarge

As a further measure of economy, the mortar used in the construction process was made onsite by local council workers using sand taken from the river, with outside contract labour used only to construct the temporary timberwork required during construction. In total, completion of the bridge took about four months.
Aesthetically, the Grand Junction Bridge was described by the media of the day as "light and graceful". It was noted that the site had been a popular picnic spot in previous years and it was felt that the new bridge would increase the natural beauty of the area - already known for its impressive rock formations and lush growth - and would encourage picnickers to return once again.

Rock face and picnic area downstream of the bridge today

Of course, an event such as the building of a new bridge - especially one which was so crucial to movement between three shires - could not go unremarked. Naturally, there was an official opening. This took place on 6th May, 1911 in the presence of such dignitaries as the Mayor of Buninyong, the presidents of each of the three shires and a large contingent of locals, all of whom were in imminent danger of losing their hats to the roaring gale which was, in the words of The Ballarat Star, "howling furiously about the high wall of rock, and stirring the sluggish waters below into unwonted activity".
So, under less than ideal conditions, the Acting Commissioner of Public Works declared the bridge open. A ribbon - placed with some difficulty by Mr Wilson - was then cut by Mrs Edgar, wife of the acting commissioner before the company enjoyed a luncheon served by the ladies. Naturally this was accompanied by the expected round of toasting and mutual back-slapping which seem to be traditional on such occasions.
Thereafter, with the exception of some discussion of the bridge with respect to the necessary adjustments to shire boundaries, the media appears to make no further mention of its existence. After the trials and tribulations over the 40 year lifespan of the previous bridge, this speaks volumes for the quality and the endurance of this revolutionary bridge which over 103 years later still survives as a tribute to the engineering skill and vision of CCP Wilson, shire engineer.